Francisella tularensis is a Gram-negative intracellular bacterium capable of causing fatal systemic disease (tularemia) in a number of mammals, including humans. The bacterium is widely recognized as one of the most dangerous bacterial pathogens known because of its ease of aerosolization, low infectious dose (ID50 < 10 organisms), multiple routes of infection, rapid disease onset, severe morbidity and mortality (33% human mortality rate if left untreated), and requirement for BSL3 laboratory manipulation. Despite concerns over the deliberate use of F. tularensis as a biological weapon, a licensed tularemia vaccine is not currently available and little is known about the correlates of protective immunity. Although phase 2 clinical trials are currently being planned for the live vaccine strain (LVS) of F. tularensis, there are well-documented concerns about LVS reversion to wild-type virulence, safety in immunocompromised individuals, and efficacy against an aerosol exposure. Given the clear need for a safe and effective tularemia vaccine, the goals of this proposal are to test the efficacy and safety of new acellular (subunit) vaccine formulations, derived from bacterial outer membrane vesicles (OMVs), in multiple animal models. In preliminary studies, we have demonstrated that OMVs provide up to 100% protection against Type A F. tularensis pulmonary challenge in mice. As such, this proposal will extend those findings by: (1) Evaluating the ability of OMVs to protect mice and rats from Type A F. tularensis pulmonary infection; (2) Evaluating the safety of OMV immunization in mice and rats; (3) Comparing the protective capacity of OMVs and LVS against Type A F. tularensis pulmonary challenge; (4) Developing and testing new recombinant protein vaccine formulations to protect against Type A F. tularensis pulmonary challenge, based upon recent data defining the components of OMVs; (5) Characterizing the immune responses induced by OMV immunization; (6) Characterizing the immune responses required to protect against and eliminate Type A F. tularensis pulmonary challenge. Taken together, the proposed experiments have enormous potential to advance a lead tularemia vaccine formulation and to test the safety and efficacy of new vaccine formulations against Type A F. tularensis pulmonary challenge.